This invention relates to a body implantable device which controls the size of a fluid flow passageway within the body, controllable from outside of the body without any direct physical connection to a control unit outside of the body.
In surgery, there may be a need for controlling, for example, the cross-sectional area of the pulmonary artery in a pulmonary arterial banding procedure. There also is a need to control the size of an atrial fenestration in the Fontan operation. Both of the above procedures are well known.
Referring to the latter procedure, the Fontan operation is performed for physiologic correction of a congenital heart condition where there is only one pumping chamber. The principle of the procedure is that blood is propelled from the right upper chamber to the lungs without the need for the pumping action of the right lower chamber or ventricle.
To accomplish this, a right atrium is isolated through the use of a septation patch made of a synthetic material such as GORETEX PTFE sheeting. Then, the isolated right atrium is connected to the pulmonary artery. In this way, the blood flows from the body to the right atrium and directly to the pulmonary arteries. The blood then passes to the left atrium, and from there is pumped into the body.
The blood flow from the right atrium to the pulmonary arteries, leading to the lungs, is totally passive in this instance, and depends on the kinetic energy supplied by the left side of the heart. As a result, there is a mild degree of blood backup on the right side of the circulation. This causes relative blood stagnation and higher systemic venous pressure, leading to undesirable extravasation of fluids from the vascular system. To counteract this, a fenestration or aperture has been provided to the synthetic septation patch applied to the heart in this operation.
The major determinants of the eventual systemic venous pressure following the Fontan operation include the pulmonary vascular resistance, the left side filling pressure, which depends on the compliance and capacity of the left heart chambers, and the degree of mechanical obstruction to pulmonary arterial blood flow or pulmonary venous return. In most cases, these determinants are abnormal in the immediate postoperative period, and tend to improve with time.
The clinical results of the Fontan procedure are critically dependent on the early and late systemic venous pressure. Markedly elevated venous pressure following surgery leads to significant extravasation of fluids from the vascular system. This results in hypotension, low cardiac output and impaired organ functions. Thus, it is important to have the capacity to intervene in the early-post operative period to assist in causing these determinants to return to normal levels to obtain the best results with the Fontan operation.
The fenestration referred to above serves as an intentional atrial septal defect or hole between the upper two chambers, to reduce pressure on the right side of the circulation for minimization of post operative complications. It is usually desirable to subsequently close this hole when the different hemodynamic parameters immediately after operation return to normal later on in the postoperative period. Such an approach is presently called the Fenestrated Fontan operation, and has yielded excellent results with markedly improved outcome of the most critically sick patients.
There are currently two clinical approaches presently practiced for fenestration in the Fontan operation. One of these approaches comprises leaving a calibrated fenestration opening in the inner atrial patch that is emplaced by the surgeon. Cardiac catheterization is performed before discharge, and a trial occlusion of the fenestration by a balloon is performed. If the patient tolerates the occlusion, the fenestration is closed with an umbrella-like device emplaced by catheter, which hooks to the edge of the fenestration and occludes the opening on an essentially permanent basis.
One problem with this approach is that the size of the fenestration or hole is not adjustable. Thus, if the hole is too big or too small, nothing can be done to adjust its size after the patient is off the cardiopulmonary bypass machine. Also, this procedure requires repeat cardiac catheterization, which of course would be desirably avoided. Also, the umbrella device used in the procedure was withdrawn from the market by the Food and Drug Administration because of breakage in a very small number of instances. Finally, once the fenestration is closed, there is no way to reopen it short of major surgery.
The other approach of Fenestrated Fontan utilizes a similar hole between the upper two chambers in the artificial septum, but uses a "purse string suture" around the hole, with one end of the string lying subcutaneously, outside of the heart. As a disadvantage of this approach, while the hole size can be adjusted by tightening or loosening the string, the adjustments are usually gross and imprecise, and require an incision of the skin and subcutaneous tissue. This, in turn, can lead to a higher instance of infection, especially if it is done on a repeated basis. Hence, this approach is close to an all or nothing situation because the fine adjustments are made only with difficulty and luck.